Implementation of PESI requires data to be combined from multiple source databases. Some of the shared fields in the source databases used different controlled vocabularies of terms. OWL DL was investigated as a mechanism to build an extensible, shared ontology of species occurrence terms that permitted the source database to continue using and extending their own vocabularies whilst formally mapping to a more generic shared vocabulary. The merits of this approach were explored and it was concluded that the building of such a complex mapping ontology probably wasn't worthwhile. The level of semantic complexity involved outweighed the costs of simply imposing a flat list of well defined terms onto data suppliers. The main problem with exiting vocabularies appear to be the overloading of terms. A candidate list of terms was proposed